There are small efforts in this direction. The biggest Presbyterian churches merged a few decades ago. The most ambitious attempt was COCU. This was largely rejected by the memberships. What has replaced it is full communion agreements.
I think there's a difference between official and de facto ecclesiologies. Official ecclesiologies often see denominations as significant. But the de facto eccelsiology of most Protestants is that they want a church with the right programs that fits their general tendencies. In effect, most members don't care what denomination a church belongs to. To them, the local church is where the action is, and denominations are organizations to facilitate their mission. This approach is official among non-denominational churches, but I think aside from conservative confessional churches, it's de facto true everywhere.
That's why the multitude of denominations doesn't bother me. Denominations are certainly useful, both for coordinating things that take efforts beyond the local church, and for providing a structure for accountability, primarily for pastors. But there's no reason there can't be many such organizations. To me the primary things are congregations and the whole Church. Thus I see unity primarily as requiring individual Christians and congregations to recognize each other as colleagues in Christ, and I'd say as a sign of that, participating in the sacraments together.
I see no denominations in the NT, but I also don't see a Pope. Rather, we have individual congregations, some aligned loosely with different teachers, and some seeing Jerusalem and maybe a regional church as having some leadership. But it's a fluid thing, not a formal division into different denominations, and not a unified hierarchy. I believe that's the reality of Protestantism, while our theory still has remnants of the Catholic hierarc
I don't see any substantial movement towards organizational unification. Rather, I see non-denominational churches as a growth area, and denominations become more modest in their claims. With some exceptions, of course.